Hydraulic oscillator and systems actuated thereby

ABSTRACT

A hydraulic actuator in the form of a fluid actuated reciprocating motor of extreme simplicity consisting of two movable elements and hydraulic circuitry. A housing encloses the two movable elements and the hydraulic circuitry is achieved within the wall of the housing. The two movable elements consist of a reciprocable piston in a cylinder and a reciprocable reversing valve in a chamber. Fluid flow through the system is continuous and uninterrupted. The reversing valve is moved alternately to its two different positions and held in such positions, by pressure differential created by different flow conditions set up at its opposite ends. These differential flow conditions are set up under the control of cylinder inlet and outlet ports which are connected to the valve chamber and are covered and uncovered by the piston during its reciprocation.

United States Patent 1 [111 3,780,622

vflgel Q 25, 1973 HYDRAULIC USCILLATOR AND SYSTEMS Primary ExaminerWilliam L. Freeh ACTUATED THEBY Assistant Examiner--Leonard Smith Attorney-Mahoney, Miller & Stebens [76] Inventor: Arthur E. Vogel, 1860 E. Cherry St., Columbus, Ohio 43205 22 Filed: June 9, 1971 [571 ABSTCT [211 App}' 151,302 A hydraulic actuator in the form of a fluid actuated reciprocating motor of extreme simplicity consisting of two movable elements and hydraulic circuitry. A LS- housing encloses the two movable elements and the 417/403, 417/553 hydraulic circuitry is achieved within the wall of the [5 CL housing The two movable elements consist of a reeip- Field 05 Search 291, 317, rocable piston in a cylinder and a reciprocable revers- 91/316, 296, 300 ing valve in a chamber. Fluid flow through the system is continuous and uninterrupted. The reversing valve Reiefelices Cited is moved alternately to its two different positions and UNITED STATES PATENTS held in such positions, by pressure differential created 1,660,528 2/1928 Rudlin 91/296 by different flow conditions Set "P at its pp ends- 1,068,649 7/1913 Hansen 91 299 x These differential flow conditions are set up under the 1,078,188 11/1913 Clement... 91/299 control of cylinder inlet and outlet ports which are 1,313,773 7/1931 Smith, 1/2 X connected to the valve chamber and are covered and 2,307,866 1/1943 Slater 91/299 uncovered the piston during its reciprocation 2,380,315 7/1945 Kilian 91/290 X 9 Claims, 11 Drawing Figures I J 8 l l l ,L l I j 8 x a WWW/4 a (I 5(/4- 6 I /Z 9 8 4-6 /9 0 PATEN I EU DECZ 51m SHEET 1 0F 7 zwmm ya 46 24 INVENTOR. ARTHUR E. l/OGEL M/f% A, ATTORNEYS PATENTEU 3.780.622

INVENTOR. ARTHUR 1 VOGEL A 77'ORNEY5 PATENTEU Z 3,780,622

' sum 3 or 7 INVENTOR. ARTHUR E. l/OGEL.

Mia 4 AT W315i PATEMEU 2 5 5 Apmupawaz-L ATTORNEYS HYDRAULIC OSCILLATOR AND SYSTEMS ACTUATIED THEREBY The oscillator of this invention is particularly useful in a system where it is desirable to have a constant source of reciprocating power. The oscillator can be operated by any suitable source of hydraulic fluid flow.

Most hydraulic oscillators in the prior art comprising a reciprocable piston and a reversing valve are operated by four-way valves. However, hydraulic reversal systems using a four-way reciprocable valve spool usually require an additional valve, such as a rotary fourway valve mechanically controlled by the cylinder movement to ensure a continuing supply of pilot pressure to the four-way valve spool so that it will not stop in the crossover position. A four-way valve spool valve used alone to control the inlet and exhaust of fluid is only permissible with compressible fluids such as air. This cannot be done with non-compressible hydraulic fluids such as oil or water, without employing saome accumulator or pressure relief valve system, to eliminate hydraulic water-hammer or shock, that is inherent with the use of such four-way valves, because of the fact that all such valves, at mid-position, are not open to allow unarrested flow.

According to my invention, I employ a hydraulic oscillator which includes merely two movable elements, mainly the piston to be reciprocated and a reversing valve which is not a four-way valve but is a more simple valve. Some of the functions of the usual four-way valve are performed by the piston cooperating with ports located at selected positions in the wall of the cylinder and connected to the valve. These ports are so located and so cooperate with the valve that there can be a continuous flow of fluid through the oscillator and the valve will be positively moved from one reversing position to another and held firmly in such position, by the creation of differential flow conditions on the opposite ends of the valve.

As long as the oscillator is subjected to uninterrupted fluid flow from a suitable source, the oscillator will function to reciprocate the piston and any movable element connected thereto. This makes the oscillator especially suitable as a power source for operating a system having a reciprocable unit which needs a constant source of power, for example, air compressors on automotive vehicles which are used to supply air for various air-operated accessories on the vehicle. I have found the oscillator to be especially useful on an automotive vehicle which has a hydraulic steering pump that can supply a constant pressure flow thereto. The air compressor operated thereby can be used in supplying the air for the air springs of vehicle levelling systems such as are shown generally in US. Pat. Nos. 3,480,288 and 3,480,293.

The oscillator of my invention will function as long as hydraulic fluid is supplied to it. A shut-off valve may be provided at the inlet or the outlet to interrupt flow so as to stop operation of the oscillator if desired. Such a valve may also be of a nature to throttle the flow of fluid to thereby vary the speed of operation of the oscillator.

The best mode contemplated in carrying out this invention is illustrated in the accompanying drawings in which:

FIG. I is an axial sectional view taken through the hydraulic oscillator showing it in its first phase of operation.

FIG. 2 is a similar view but showing the oscillator in its second phase of operation.

FIG. 3 is a similar view but showing the oscillator in its third phase of operation.

FIG. 4 is a similar view but showing the oscillator in its fourth phase of operation.

FIG. 4A shows a hydraulic oscillator similar to that shown in FIG. 4 but with a different arrangement of the two movable elements.

FIG. 5 is an axial sectional view showing the oscillator of FIGS. 1 to 4 combined with an air compressor which it actuates.

FIG. 5A is a fragmentary axial sectional view showing the piston of the compressor of FIG. 5 with its valve plate in a different position.

FIG. 6 is a fragmentary axial sectional view showing an on-off control valve incorporated in the oscillator.

FIG. 7 is a schematic perspective view showing the oscillator-driven compressor embodied in a vehicle suspension system.

FIG. 8 is an axial sectional view through an air unloader valve used in the system.

FIG. 9 is a schematic perspective view showing the oscillator-driven compressor incorporated in a vehicle suspension system having a different type of levelling valve.

A general description of the hydraulic oscillator will aid in understanding the specific description to follow.

The hydraulic oscillator of my invention comprises a housing having a main pressure inlet and an exhaust outlet with a cylindrical chamber formed within the housing having a movable element in the form ofa piston slidably mounted therein. The cylinder or chamber has a first pair of axially spaced pressure inlet ports connected to said main inlet port and being positioned so that both are closed in an intermediate position of said piston but are alternately opened as the piston completes its stroke in opposite directions so as to alternately supply pressure in the chamber at the opposite faces of the piston. The cylinder has a second pair of axially spaced exhaust ports which are closed in an intermediate position of said piston but are alternately opened to opposite ends of said cylinder as the piston completes its stroke in opposite directions, one of said inlet ports and one of said outlet ports at the respective ends of the cylinder being exposed by the piston simultaneously. A third pair of ports is provided and these ports are at the respective ends of the cylinder and serve alternately as inlet and exhaust ports. A second movable element in the form of a reversing valve is provided and specifically is a spool mounted for reciprocation within a valve chamber or bore formed within said housing. This valve has a single fluid passage formed intermediate its ends. The main pressure inlet and said pair of pressure inlet ports for said cylinder are always connected to said fluid passage of the valve by suitable passages. One of said second pair of axially spaced exhaust ports is constantly connected to one end of said valve chamber and the exhaust outlet and the other is constantly connected to the other end of said valve chamber and the exhaust outlet by suitable passages. The ports of said third pair which are at the ends of the cylinder are connected by suitable passages to said valve chamber at axially spaced ports which alternately respectively communicate with said fluid passage of the valve as it completes its stroke and one end of the valve chamber at which are both closed when the valve is in an intermediate position.

Assuming the valve is at one end of its stroke, pressure fluid is supplied from the inlet through the valve passage to one of the end ports of the cylinder into that end of the cylinder and at the same time fluid is exhausted from the opposite end of the cylinder, through the end of the valve chamber and out through the outlet. At the same time, both of the cylinder exhaust ports are closed by the piston and the opposite end of the valve is therefore not subjected to any dynamic flow of exhaust fluid as the other end is. Thispressure differential on the opposite ends of the valve created by dynamic exhaust flow on the one end and static conditions on the other end, results in firmly keeping the valve in this end position during this first phase of the operation.

However, when the piston reaches the extent of its movement in the one direction, one of the pressure inlet ports of the cylinder is exposed by the piston and at the same time, the corresponding exhaust port is exposed by the piston. This permits pressure fluid to flow from the main inlet into the same end of the cylinder through the alternate branch passage extending from the main inlet to the said exposed cylinder inlet port. At the same time fluid exhausts from this end of the cylinder to the opposite end of the valve and on out to the outlet creating a dynamic flow condition at that end of the valve. The other pressure inlet port and exhaust port of the cylinders are closed at this time and there is, therefore, no dynamic flow on the other end of the valve. This pressure differential will cause the valve member to move away from its end position toward the other end of its chamber. This movement causes the valve to immediately close both ports in its chamber which connect with the end ports of the cylinder. This prevents supply of fluid to either end of the cylinder by these ports but fluid is still being supplied to the one end by the alternate branch passage including the cylinder inlet port exposed by the piston.

As the valve completes its reversing movement, it connects the main inlet through its fluid passage, and the other cylinder end port to the other end of the cylinder supplying pressure therein to move the piston in a reverse direction. The valve will be held firmly in this position by the pressure differential created thereon by the exhaust of fluid from the other end of the cylinder through the port at that end which is connected to the other end of the valve chamber. The end of the valve chamber toward which it moved is not subjected to dynamic flow since the cylinder exhaust port connected thereto is closed by the piston. Also, the other exhaust port of the cylinder is closed by the piston at this time as are the two pressure inlets of the cylinder.

The valve will stay in this position until the piston completes its movement in the indicated direction at which time it will again be moved in a reverse direction. As the piston completes its movement, the other pressure inlet port and corresponding exhaust port are exposed by the piston. This sets up a branch alternate passage from the main inlet, through this cylinder inlet port. The flow of fluid through the exposed cylinder exhaust port into the other end of the valve chamber creates dynamic pressure thereon whereas there is static pressure on the opposite end due to the fact that the cylinder exhaust port connected thereto is covered. This will create movement of the valve member in a reverse direction, immediately closing both of the ports of the valve chamber which connect to the cylinder end ports. The cycle of operations will be completed over and over and at no time will fluid flow from the main inlet to the main outlet be interrupted.

The oscillator will function as long as hydraulic fluid is supplied to it. A shut-off valve may be provided at the inlet or the outlet to interrupt flow so as to stop operation of the oscillator. Such a valve may also be of a nature to throttle the flow of fluid to thereby vary the speed of operation of the oscillator.

With particular reference to the drawing, I have illustrated in FIGS. 1 to 4, inclusive, the hydraulic oscillator of my invention. The following is a detailed description of its structure and function. it is a fluid actuated reciprocating motor of the most extreme simplicity using only the irreducible primaries of two movable elements and a hydraulic circuitry. It is novel and simple compared to prior art. It will be noted that the circuitry is achieved within the wall of the housing that contains the two movable elements. The housing is number 1 generally in FIGS. 1, 2, 3 and 4. One of the above mentioned elements is the driven element, or piston generally indicated number 2. The other element is the flow sensitive reversing valve and is generally indicated number 3. The housing is provided with an inlet fitting 4, and an outlet fitting 5, to provide forthe admission of fluid to, and the exhausting of fluid from the motor. The housing is also provided with end caps 6 and 7. in addition, the housing is also fitted with a number of press fit ball plugs. These balls merely plug access holes that were necessary to create intersecting drilled passages. They are indicated by the number 8. Within the housing there is another press fit ball, number 9, which is a hydraulic fluid exhaust flow restrictor.

The hydraulic motor operates as follows: H6. 1, fluid enters inlet fitting 4, to passage 21 to chamber between valve spool lands 1%, of said spool 3. Now it will be noted that reversing valve 3, is in the right hand position and resting against the end cap 6. When the valve 3 is in the right hand position, passage 21 is open to passage ll via spool land cavity 10, and the fluid flow is continued through passage 12 and port 13 into chamber 14. Fluid pressure acts against surface 15 and 15A of driven element 2, driving it and its output shaft 16 to the left position. It can now be noted that the output shaft 16 is moving also outside the motor and can be called upon to perform useful mechanical work of wide scope. While the driven element 2 is moving from right to left, hydraulic fluid is being expelled from chamber 17 through port 18 into passages 19 and 20 and into chamber 22 out through port-passage 29 through passage 24L past ball restrictor 9 and finally out through passage 25 of exhaust fitting 5. It will be noted that during this phase of the motors operation, the fluid exhaust flow pressure is greater in chamber 22 than it is in chamber 23 because of the dynamic flow within chamber 22 and conversely because of the exhaust circuit conditions in chamber 23 and passage 24R which are static. The resulting pressure differential between chambers 22 and 23 serve to keep the reversing valve 3 firmly in its right hand position during the motors first phase of its operation. This phase continues until the driven element 2 has reached its complete leftwardly travel and is against end cap 7, as shown in FlG. 2. At this point, phase 2 begins in the following manner: Referring further to P16. 2, it will now be noted that a port 26, has been opened by the leftward movement of the driven element, shown generally at 2, and specifically by its surface 15, said surface being left of port 26. Port 26 is a fluid pressure source to chamber 14, and is being fed by inlet fitting 4 and passages 2H and 28 through the wall of the housing I, further described as intersecting drilled passages disposed radially and longitudinally within the wall of the housing. Note that access drillings perpendicular to, and through the wall of the housing, make not only passages but also holes in the exterior of the wall that must be plugged. These holes are plugged with press fit ball plugs 8, as previously partially described. Further referring to FIG. 2, it will be noted that surface of driven element 2 has exposed another port 3t which in turn leads through passages 31 and 311R to chamber 23. At this point, it is further explained that all fluid flow has ceased through port 18, and the fluid in passages 19 and 20 is now static. Note that during this transition phase the fluid flow is not interrupted, but is alternately channeled through branch passage 23, through port 26, into chamber I4, to port-passage 30, through passages 31 and SIR to chamber 23, through passage 24RR and 24R past ball restrictor 9 through passage and out of the motor through fitting 5. Further, note in FIG. 2 that the dynamic fluid exhaust flow is now through chamber 23, and that chamber 22 now experiences the static exhaust circuit condition commonly shared with port-passage 29 and passage 24L.

It should be further noted that the viscous drag of fluid flowing through chamber 23, port-passage 24RR and passage 24R is further augmented by the presence of ball restrictor 9, to effect sufficient pressure differential between chambers 23 and 22 to insure the complete displacement of reversing spool valve 3, from its right hand position, through the dead center position, to the completed left hand position, as shown in FIG. 3. With continued reference to FIG. 2, it is further pointed out that reversing spool 3 is able to move from right to left, because chamber 22 is always open to port-passage 29, thereby allowing for the discharge of fluid through passage 24L and out of the motor through passage 25 of exhaust fitting 5. Thus it can be said that reversing spool valve 3, is urged to move to the left, because of the aforementioned dynamic pressures in chamber 23, and permitted to move to the left by the free escape circuits from chamber 22 to the outlet. Now that reversing spool valve 3 is in the extreme left position the motor now begins phase three as follows: Fluid flow continues to enter the motor through fitting 4, passage 21 into and around cavity 10 of reversing spool valve 3, through port-passage 20, through passage 19, out port 18, into chamber 17. The fluid pressure acts upon surface 41 and 41A of driven piston element 2, forcing it to move from left to right. It might be noted that 42A, 42B and 42C are 0 ring seals. Now to continue with the description of the motors function with respect to FIG. 3. It will be noted that the exhausting of fluid from chamber 14 is achieved via port-passage I3, passages 12 and 11 into chamber 23 and then out through port-passage 24RR and 24R, past ball restrictor 9, and out of the motor through passage 25 of exhaust fitting 5. Note that now the dynamic exhaust conditions continue to exist in chamber 23, thereby biasing spool valve 3, keeping it firmly in its extreme left hand position during this phase of the motors operation.

With continued reference to FIG. 3, this third phase also serves to pull the work rod 16 back into the housing. It will be further noted that the pulling motion can also do useful mechanical work per se. The third phase of the motors operation ceases when the driven spool 2 reaches its extreme right hand position, as seen in FIG. 4. Simultaneously, all fluid discharge from chamber I14 ceases to flow out of port-passage I3, and passages ll2 and l I. Fluid flow is maintained, however, uninterrupted through the motor, in the following manner. With continued reference to FIG. 4, as the driven spool element 2 moved to the extreme right hand position, its left end surface 41, opened port 45 and port 46. Note that port 45 is an alternate pressure port similar to port 26 as shown in FIG. 2. However, this port 45, in FIG. 4, now having been opened, allows the fluid to continue through the motor via inlet fitting 4, passages 21 and 28, through this port 45, into chamber 17, then through port 46 into passage 24LL and 24L, past ball restrictor 9 and out of the motor through passage 25 of exhaust fitting 5. This dynamic condition has viscous drag through passage 24L and around ball restrictor 9. The resulting pressure is reflected as a differential bias to the reversing spool 3. This pressure bias goes through passage-port 29, into chamber 22, and chamber 23 will now discharge the fluid contained therein through port-passage 24RR, passage 24R and out of the motor through passage 25 of exhaust fitting 5. Because the movement of reversing spool valve 3 from its left hand position, through dead center position, to the extreme right hand position, causes the displacement discharge in chamber 23, when reversing spool valve 3 reaches the extreme right hand position, the fourth phase has been completed, and the reversing spool valve is again at the position shown in FIG. I and the motor is now back to phase I of its operation. As long as fluid is delivered to inlet fitting 4, and as long as the flow of fluid is not closed off after leaving exhaust fitting 5, this reciprocating hydraulic motor will continue to operate indefinitely as herein described. However, this is not to say that said motor could not be externally stalled by a resistance to the movement of work rod 16, if insufficient hydraulic pressure was not available to inlet fitting 4.

It should be especially noted that at no time during the transitions of reversing valve 3, did any cessation of hydraulic fluid flow occur, with respect to such flow entering or exhausting from fittings 4 and 5, respectively. Not for even a micro-second, as would be the case if a four-way spool valve were employed. A four-way spool valve, controlling both the inleting and exhausting of fluid is only permissible with compressible fluids such as air. It is known to the art of hydraulics that this cannot be successfully done, with the non-compressible fluids such as oil or water, without employing some accumulator or pressure relief valve system, to filter out hydraulic water hammer to shock, that is inherent with the use of such four-way valves, because of the fact that all such valves, at mid-position, are not open to allow unarrested flow. In contrast, this reciprocating hydraulic motor has the feature of having always open circuitry throughout all phases of its operation.

Referring to FIG. 4a, a modification of the oscillator is shown in which the housing IX is of different form to make it possible to have a movable element or piston 2A with piston rods ML and MR extending from opposite ends thereof through hydraulic seals 2413. This is desirable for installations having units to be actuated by each of the piston rods. Because of the oppositely extending piston rods, it is necessary to dispose the valve 3A with its axis in parallel realtionship to the axis of the piston 2A rather than in alignment, as in FIG. 4. All of the equivalent chambers, ports and passages in this example are designated by the same numerals as in FIG. 4 with the addition of the subscript character A. A detailed description of the operation of this form of the oscillator is not deemed necessary since it will be identical with that previously described.

It will be apparent from the above description of both examples of this hydraulic oscillator or motor that it consists basically of two movable elements, namely, the main movable pressure-actuated element, illustrated in these examples as a piston, and the movable control element or valve, illustrated in these examples as a spooltype valve, the two elements being located in separate chambers. The spool-type valve divides the main valve chamber, in which it axially reciprocates, into an inter mediate fluid-passage chamber and opposed end fluidpassage chambers. The main pressure inlet for hydraulic pressure is always connected to the main fluid passage chamber of the valve. Fluid continuously flows from the main inlet to the main outlet through both end chambers of the valve, under the control of movement of the piston in such a manner that dynamic flow conditions are created at one end of the valve and static conditions are created at the other end of the valve, alternately, resulting in the reversing movement of the valve. Thus, the main inlet is always connected to the main outlet and there is a continuous flow of hydraulic fluid through the oscillator.

The hydraulic oscillator previously described can be used as a power unit to perform various functions. One example is shown in FIGS. 5, 5A and 6 where it is shown connected to an air compressor so that the reciprocating movement of the driven element will work the piston of the compressor. As seen in FIG. 5, the end cap 7a is fitted to main housing 1A and an air compressor end cap 78 is screwed on main housing A1, at the screw threads generally indicated at 7T. Note that the air compressor housing 78, has a shoulder 78, which holds endcap 7A firmly in place against main housing 1A. Further note that end cap 7A is the end of the hydraulic portion, and further represents the beginning of the air compressor section. Thus, it should more correctly be referred to as divider cap 7A. It can be further seen that said divider cap 7A contains hydraulic seals 42A and 428, as was the case in FIGS. 1 through 4. Still referring to FIG. 5, note the oscillating shaft 16 connects to an air compressor piston, generally indicated at 50. The connection or attachment is achieved by a bevel headed allen cap screw 51, which is screwed firmly into shaft 16 thereby holding air compressor 50 to shaft 16. It will now be noted that the oscillating motion of hydraulically driven spool 3 is causing the likewise oscillating motion of the air compressor piston through the rod 16. Referring to FIGS. 5 and 5A, it will also-be noted that the bevel of the attachment screw 51 further extends its surface to retain an air piston inlet valve 52 within air piston 50. However, it should be pointed out that screw 51 does not hold the inlet valve plate 52 tightly within air compressor piston 50, but rather, screw 51 permits limited outward movement of valve plate 52 from the seating surface 58 of air piston 50 as seen in FIG. 5A. In FIG. 5, valve plate 52 is shown in sealing contact with air piston surface 58 of piston 50. The sealing is assured by the presence of O rings 53 and 54 shown in FIGS. 5 and 5A. As seen in FIG. 5 and 5A, "0 ring 53 stays affixed to valve plate 52, by virtue of the fact that it was assembled, stretched over the diameter of inclined plane surface 59 of valve plate 52. Similarly, O ring 54 has been exapnded into inclined plane surface 60, of valve plate 52. This 0" ring 54 was selected slightly larger in diameter than the diameter of inclined plane surface 60 of valve plate 52. This resulting slight compression of the diameter of the O ring 54- maintains it affixed within the valve plate 52. Further, in FIGS. 5 and 5A, another seal 57 is seen in a groove at the outer diameter of air piston 50 and it is in a sliding engagement with surface 7C of air compressor housing 713.

It can now be readily seen how the hydraulic flow of fluid compresses air, as follows: As described in FIGS. 1 through 4, hydraulic flow causes shaft 16 to oscillate left and right. Now then, as seen in FIG. 5, atmospheric pressure enters the air compressor section as fiiter 65, through passage 66, through hole 7H and then into chamber 62, as elements 2, 16 and 50 move from right to left, the air thereby following piston 50. At the same time, while the air piston moves from right to left, the air in chamber 61 is being compressed and escapes around spring loaded exhaust valve 63 and out of the air compressor, through hole 64 of fitting 68. FIG. 5A shows the air compressor inlet stroke. Shaft 16 and piston 50 are now moving left to right, creating a slight vacuum in chamber 61. Atmospheric pressure in chamber 62 passes through radial holes 55 in piston 50 and pushes valve plate 52 open, to allow air to pass around seal 53, through clearance passage 56, and into chamber 61. After this left to right stroke is completed, the air will then be compressed in chamber 61, as previously described. It will be noted that the seating and unseating of valve plate 52 is accomplished by both the forces of inertia and air flow at each reversal stroke, and that no inlet valve biasing spirng is used. Valve plate 52 has sufficient mass and area to seat and unseat. This feature insures the complete atmospheric charging of chamber 61, thus each compression stroke enjoys maximum efficiency.

Referring now to FIG. 6, an on-off valve is shown and indicated generally at 70. This valve will stop the oscillator when the air compressor builds up a selected high pressure and will start it again when the pressure drops to a selected low pressure. This on-off valve is not an essential part of the hydraulic oscillating air compressor, as clearly, it could be employed in a fluid circuit outside of the wall of the oscillator with exactly the same control circuit effect. However, as shown in FIG. 6, the on-off control valve is shown spring loaded by spring 71 to the right hand position and up against end cap 6. It will be noted that valving land 70V of this on-off control valve element 76 is shown blocking hydraulic port 21A from communicating with exhaust port 25 and outlet fitting 5. This is the on position, wherein the oscillator functions responsive to hydraulic fluid flow as described in detail with respect to FIGS. 1 through 4.

With continued reference to FIG. 6, the on-off control spool valve element 70 can be placed in the off" position by the application of an external fluid pressure signal, supplied by control line valve 75. Upon application of said such external pressure signal from control line 75, valve element 79 would be shifted to compress spring 71, as such signal would enter chamber 72 by way of passage 73 of fitting 74A. An ring seal, 708, is shown to confine such signal pressure to chamber 72 and to the right side of valve element 70. When the signal pressure in chamber '72 is of a greater value than the force of spring 71, valve element 70 will be forced to the left position (not shown). However, it can be immediately seen that the spool valve land channel section 7IIC of valve element 7% would be in alignment with passages 21A and 25, thereby permitting a new short exhaust circuit directly through the motor. This alternate by-pass circuitry provides for stopping the motor from oscillating, while still maintaining full hydraulic volume flow through the unit as follows: Hydraulic fluid enters fitting 4 of FIG. 5, flows through passage 21, around channel 10, of reversing spool 3, into passage 21A of FIG. 6, around channel groove 70C of on-off control spool element 70, into passage 25, and out of the motor through fitting 5 of FIG. 6. This condition maintains the oscillating compressor shut off so long as the off signal pressure is applied to control line 75. It will be further understood, however, that the release of control pressure within line 75 to atmosphere would cause the spring 71 to return the control valve 79 back to its right hand position, as shown in FIG. 6, at which time outer land 70V of control valve 70 blocks alternate passage 21A whereby oscillating resumes. The above mentioned control of pressure signals to line 75 will be further described in the following control system of the vehicle suspension of FIGS. 7 and 8 and in still another control system for vehicle suspension in FIG. 9 wherein no hydraulic shut off is required.

As indicated, the hydraulic oscillator of my invention can drive an air compressor and this compressor may supply air to the air springs of a vehicle leveling system. It may receive constant pressure hydraulic fluid from the power steering hydraulic pump on the .vehicle. A control system for vehicle suspension of this type is shown in FIG. 7 and operates as follows: The source of power is the vehicle propelling engine from which a fan belt 80 is drive. It, in turn, rotates drive pulley 81 of a conventional automotive power steering hydraulic pump, generally indicated at 82. All such pumps incorporate a flow control means generally indicated at 83, and such flow control means $3 causes the pump to discharge at an almost constant gallonage per minute, regardless of the revolutions per minute of the driven pulley 81. Such resulting G.P.M fluid outputs are in the 2 G.P.M. category, as is well known to the art.

A typical automative power steering system is operated as shown in FIG. 7 by the fluid pumps output being conducted through pressure line 84? through fitting MF, into the control valving 85C of the steering mechanism. Such control valving 85C is of course directed by steering wheel 38C. During all normal operations of the steering mechanism 85C, hydraulic fluid is exhausted from it through fitting 87F and then back to the fluid pump reservoir fitting EXR4. However, it will be observed that the system as shown in FIG. 7 places oscillator unit IA, and oil cooler unit 88, in a fluid series circuit with exhaust fitting 87F and fluid pump reservoir fitting EXR4 via first exhaust line EXI, oscillator inlet fitting 4, through oscillator 1A, out of oscillator outlet fitting 5, through exhaust line EXZ, through oil cooler 88, through exhaust line 8X3, and back to the fluid pump reservoir, through fitting EXR4. Therefore,

it will be noted that oscillator IA is supplied with hydraulic fluid power as stated, in series with a conventional vehicle power steering system and will oscillate as described in FIGS. 1 to 6.

As oscillator IA functions, it will be noted that in FIG. 7, the air for this vehicle control system enters the system through inlet filter 65, through intake line 66, into air compressor unit 7 B and out of the compressor unit through compressor fitting 68, in the manner described in FIGS. 5 and 5A.

Air delivery pressure line 89 conducts the compressed air to high pressure reservoir RI through fitting 89F. The amount of air pressure in RI can be observed by a pressure gauge PGI fitted to the tank RI. A snap action air switch CI is similarly fitted to the high pressure reservoir R1, at fitting 90. The control switch CI is incorporated into this system, to provide the on-off signals to control line 75. Such signals in line shut off and turn on the oscillator, as previously described, by element 70 in FIG. 6. The internal functions of snapswitch Cll are fully shown in FIG. 8. However, for now, continued reference is made, with respect to FIG. 7. Snap-switch Cl causes the oscillating compressor to maintain the air pressure in the high pressure reservoir Rll between and PSI. regardless of the intermittent consumptions of the rest of the system. Two air lines, 91A and 91B, are shown leading from the high pressure reservoir R1. 91A leads to a conventional air pressure regulating reducing valve C2, the function of which is to maintain a pressure in the low pressure reservoir R2 at 15 P. S. l. The resulting stable power source within the low pressure reservoir R2 can operate a host of diaphragm and bellows operated accessories. In the past, such diaphragms and bellows have been actuated by atmospheric pressure toward partial and variable vacuums, produced within the vehicle engine intake manifold. It is well known to the art that no vacuum is available during acceleration and also, in places of high altitude, the atmospheric pressure is less than at sea level. Furthermore, flexible vacuum tubing is expensive because of its thick wall, and it normally tends to collapse, especially at turns, and at locations where higher than ambient temperatures are encountered, such as in the engine compartment and near exhaust pipes etc. In contrast, when control diaphragms and bellows are actuated by positive pressures, above atmospheric pressure, a very inexpensive, thin walled, plastic tubing can be employed and the positive pressures therein tend to straighten out the kinks, by rounding the radii. This resulting control system reliability is further augmented by a considerable reduction in plumbing expense.

Automatic leveling of the vehicle's suspension system is achieved from the high pressure power source RI, as shown in FIG. 7, by piping said power through line 918 to a conventional leveling valve C3, which is mounted upon the vehicle's sprung weight frame 97 and suitably connected to be actuated by the relevant movement between said sprung weight 97 and unsprung weight of vehicle axle 96, by means of connector links 98 and arm valve lever 99. Further, in FIG. 7, it will be noted that suspension leveling valve C3 operates as described in previous US. Pat. No. 3,480,293. I. e., if the air springs 93 and 95 require air, leveling valve C3 supplies it to them via lines 92 and 94l and if air springs 93 and 95 require a reduction of pressure (like when the vehicle is unloaded), the leveling valve C3 will expel to atmosphere through outlet port C4 the required amount of air from air springs 93 and 95 to maintain the vehicle level.

Snap action control valve C1 is shown in detail in FIG. 8. Its function is to shut off the oscillating compressor when the desired predetermined air pressure has been pumped into the high pressure reservoir R1 of FIG. 7. In FIG. 8, inlet fitting is shown at 90, the pressure within reservoir R1 is conducted into passage 104 of lower housing of unloader valve C1 and is sealed by the presence of O ring 103 which is in the inclined plane slot 105 of piston slug 101, and because spring 100 is holding piston slug 101 down against sealing surface 106 of lower housing C1. The aforementioned seal is established so long as the air pressure in passage 104 does not overcome the force in spring 100. It will be noted that adjustment nut CIA has external threads 107 which are partially screwed into internal threads 108 of upper housing ClB. This provides for the adjustment of nut C1A to change the loading or spring 100. Further, in FIG. 8 it will be noted, piston slug 101 is slightly smaller in diameter than the internal bore of lower housing C1. The resulting clearance path 111, along the surface of piston slug 101, provides a very low volume exhaust path from line 75 to atmosphere, as follows: As shown in FIG. 8, line 75 is able to exhaust the fitting 748 into port 109 along clearance path 111, around ring seal 102, into spring cavity 110 and then out to atmosphere, through hole 112 of adjustment nut ClA. This of course causes the oscillating compressor 1A to run, as previously described in FIG. 6, with specific reference to on-off control spool valve element 70.

With continued reference to FIG. 8, it will be understood that the pressure in port 104 of lower housing C1 is effectively working upon the circumference area of the circle. lip of O ring 103, resting on seal surface 106.

As the oscillating air compressor builds up pressure in the reservoir R1 to the desired predetermined maximum, such pressure is also in passage 104. This rising pressure, being approximately 125 P.S.l., tends to compress spring 100 slightly, as piston slug 101 and its self contained 0 ring seal 103,-are forced away from seal surface 106. The moment leakage occurs between seal 103 and surface 106, compressed air flows along clearance path 111, around upper O ring seal 102, cavity 110, through hole 112 to atmosphere. The clearance path 111 is fitted so closely in the bore of housing C1 as to allow only a minute leak to atmosphere. Therefore, it is to be noted that the reservoir pressure is not acting upon the entire end diameter of piston slug 101. This area being much greater than the area of the lip diameter of O ring seal 103 and the pressure working against this larger area, causes piston plunger 101 to snap away from housing surface 106, until alternate O ring seal 102 strikes and rests upon alternate sealing surface 113, of upper housing ClB, thereby cutting off the flow of air to atmosphere. It will be further noted that O ring 102 is clinging to piston slug 101, by virtue of it having been installed in inclined plane surface 114, in a slightly stretched state, larger than its manufactured diameter. With continued reference to 0" ring seal 102, its lip diameter is larger than that of O" ring seal 103. As previously explained, the lip area of seal 103 responded to the rising pressure in passage 104 and caused the piston slug 101 to move in opposition to spring 100, followed by the snap movement of piston slug 101 to alternate seal surface 113, and now the same reservoir pressure holds the piston slug 101 to the alternate contact surface 113 by virtue of the larger effective area, provided by the piston slug 101, by the larger lip diameter of the O ring seal 102. While the piston slug 101 is up against the alternate seal surface 1 13, some of the compressed air in the reservoir can flow to the on-off hydraulic by-pass control element within the wall of the oscillator compressor, shown in FIG. 6, thereby shutting the compressor OFF and achieved in the following manner. With continued reference to FIGS. 6, 7 and 8, in FIG. 8, the piston slug 101 being against alternate seal surface 113, a space exists between lower 0 ring seal 103 and surface 106 of lower housing C1, thereby completing a passage circuit, from reservoir R1, through passage 104, across surface 106, to clearance passage 1 l 1, through passage 109, out fitting 748, through line 75, into fitting 74A of FIGS. 7 and 6, through passage 73 and finally into chamber 72 of FIG. 6. The control pressure now in chamber 72 of FIG. 6 pushes hydraulic by-pass valve element 70 to the left, causing the oscillating air compressor to cease supplying the reservoir R1 with compressed air. The larger diameter of O ring seal 102, from the diameter of 0" ring seal 103, furthe provides for the maintainment of contact between piston slug 101 and alternate seal surface 113 and against the efforts of spring 100, until such time as the reservoir pressure falls substantially below the pressure value that was required to move the piston slug 101 away from the first mentioned seal surface 106. Thus, an on-off pressure differential does exist. However, when the system pressures in the reservoir R1 of FIG. 7 and in passage 104 of FIG. 8 does, in fact, fall below the stated differential, as is the case, because the system of FIG. 7 does consume pressurized air from R1, via line 91A and 913, the piston slug 101 will quickly return to the seal surface 106 of lower housing Cl. This occurs at the time when the reduced air pressure force in passage 104 of FIG. 8 can no longer hold piston slug 101 to upper sealing surface 113 because of the opposing force exerted by spring now being greater. The moment piston slug 101 does return to the first sealing surface 106 of lower housing C1, the oscillating resumes functioning in the manner previously described, thereby replenishing reservoir R1. It has been discovered that the desirable range of pressure differential to maintain in reservoir R1 is 100 to P.S.I. However, a different maximum, minimum and differential can be quickly established by adjusting the threads 107 of nut C1A deeper, or out of the internal threads 108 of upper housing ClB, as seen in FIG. 8. Similarly, internal threads of upper housing C1B engage external threads 116 of lower housing C1, providing a partial pressure differential adjustment. The primary differential is established by the design selection of the different circumferences of the 0" ring seals, 102 and 103, that are applied to piston slug 101 and the long close clearance passage 111. I have further provided the piston slug 101 of FIG. 8 with a split step piston ring 118 in piston groove 117. This augments the retardation of flow along close clearance path 1 1 1. This insures a very reliable snap action when used to control compressors developing a small rate of pressure rise, as in passage 104.

In FIG. 9, l have illustrated my oscillator driven air compressor incorporated in a vehicle levelling system of the general type shown in US. Pat. No. 3,480,288 in order to obtain a constant air supply for that system. This vehicle leveling system is shown in FIG. 9 as two air inflatable, rear shock absorbers, shown generally at 229A and 229B. They are mountable between the vehicle chassis and the left and right rear axle housings. Upper mounting holes are provided at 200L and 200R, suitable for mounting to the chassis. Similar mounting holes 201L and 201R are provided at the lower end for mounting to the left and right rear axle housings. It will be noted that these shock absorbers are provided with flexible rubber boots 202L and 202R which comprise chambers that can assist the vehicles rear springs in suspending the vehicle level when more passengers or luggage are added to the vehicles gross weight.

The leveling system functions as follows: The engine propelling the vehicle rotates pulley 81 via fan belt 80 thereby driving hydraulic steering pump 82. The flow control means 83 of said pump 82 discharges hydraulic fluid at a constant rate regardless of the varying engine speed, as previously described. The power steering control unit 85C receives the hydraulic power from the pump 82 via line 84F and inlet fitting 84F. The flow continues out of the power steering unit through fitting 87F through exhaust line EX], to, and into, oscillating air compressor 1A via inlet fitting 4. Hydraulic fluid flow continues out of the oscillating air compressor at fitting 5 and through exhaust line EX2, through oil cooler 88, through exhaust line EX3, and then back into the pump reservoir, through fitting EXR4, thus both the power steering unit 85C and oscillator 1A are supplied with fluid energy. As the oscillator 1A runs, the air compressor section 7B functions also and air enters it at intake 65, via line 66. Compressed air leaves the oscillating compressor via fitting 68 and line 89A to and through pop-off valve 210, continuing to inflatable shock absorber 229A, via line 89B. Air continues to flow from shock absorber 229A to shock absorber 2298, via line 39C. Air then flows out of shock absorber 2298, via line 89D, to and through adjustable snorkle leveling valve shown generally at 207. The snorkle leveling valve 207 exhausts to atmosphere via its port 203 when the vehicle is unloaded and the rubber chamber wall 202R is above and uncovering port 203, as described more in detail in said patent. The line 898 may be provided with a relief valve 210 having an exhaust port 212 to prevent the development of excessive pressure therein in case of overloading of the vehicle.

lt will be apparent from the above description that the oscillator of my invention provides a simple means, where a source of hydraulic fluid flow is available, for obtaining a constant reciprocating movement of a movable element. This element may be a reciprocable element in any of various units. For example, it may be, as described above, the piston of an air compressor which is used to control various air-actuated accessories on a motor vehicle. Such accessories may include the vehicle levelling systems described above. Such vehicles usually have a hydraulic steering pump and this pump can be used as a constant source of liquid flow to the oscillator. The result will be a constant source of air pressure for the system as compared to the varying source usually provided by a manifold-actuated vacuum system which varies widely with the acceleration of the vehicle.

As indicated, the hydraulic oscillator is a simple hydraulic unit comprising a housing containing a movable element or piston to be reciprocated and a slidable spool-type valve to serve as a reversing valve. All fluid passages of the oscillator preferably are formed within the wall of the housing as described. The reversing valve is a simple valve spool with a single passage formed between a pair of axially spaced lands on the valve. The simplicity of the valve results mainly from provision of control ports provided in the wall of the cylinder in which the piston is disposed for reciprocation. These ports are so connected to the chamber in which the valve reciprocates and to the main inlet and outlet of the housing that a differential flow effect is set up at opposite ends of the valve spool to cause it to move at the proper instant into a reversing position and to stay in that position until that instant when it is to move in the opposite direction to its other reversing position. The piston is moved in one direction by pressure supplied through a passage from the main inlet through the valve passage and through a port which is never covered by the piston. When the piston reaches the extent of its movement in that direction, it uncovers ports which establish an alternate branch passage from the main inlet through that end of the cylinder to the opposite end of the valve spool, creating a pressure differential thereon which causes it to move into its opposite or reversing position. This action is repeated over and over as long as fluid flow is supplied to the oscillator. Because of the design of the reversing valve and the ports which it controls, and the design of the piston and the ports it controls, the flow of fluid through the oscillator is never interrupted.

Having thus described my invention, what is claimed 1. A hydraulic oscillator comprising two movable elements only consisting of a reciprocable piston in a cylinder and a reciprocable reversing valve in a chamber, means for creating differential flow conditions at the opposite ends of the valve in the valve chamber, said means including inlet and outlet ports in the cylinder which are connected to the valve chamber and are covered and uncovered by the piston during its reciprocation, said movable elements disposed in a housing having a main inlet and outlet with the piston reciprocable in the cylinder formed in said housing and the valve member reciprocable in the chamber formed in said housing, said valve member having a single fluid passage formed intermediate its ends, said valve member fluid passage being constantly connected to said main inlet, said valve chamber having opposed ends constantly connected to said main outlet, a passage between said valve fluid passage and one end of said cylinder and a passage between one end of said valve chamber and the other end of said cylinder, said valve member in its extreme end positions serving to connect alternately the main inlet to one of said passages and disconnecte the other therefrom, the disconnected passage being connected to one end of the valve chamber and to said outlet to create a dynamic pressure differential thereon to hold it in said extreme position, and a pair of cooperating inlet and outlet ports in each end of the cylinder alternately exposed and closed by the piston in its stroke in opposite directions for creating at the end of each stroke an alternate branch passage from the main inlet, through the valve member fluid passage and the end of the cylinder with the exposed ports to the opposite end of the valve chamber to create a dynamic differential pressure thereon to cause it to move in a reverse direction.

2. A hydraulic oscillator comprising two movable elementsconsisting of a main movable pressure-actuated element and a movable control valve member located in a main chamber and a valvechamber respectively; said main movable member disposed in its chamber for movement therein and dividing the main chamber into opposed end chambers; said movable valve member disposed in its chamber and of such form as to divide the valve chamber into a single intermediate fluidpassage chamber and opposed end fluid-passage chambers; a main pressure inlet passage always connected to the intermediate fluid-passage chamber of the valve member in all its positions of movement; a main outlet passage always connected to both the opposed end fluid-passage chambers of the valve chamber; and fluid passages connecting the end chambers of the main chamber and the end fluid-passages of the valve chamber under the control of said pressure-actuated element to create dynamic flow conditions in one of said end fluid-passage chambers of the valve and static conditions in the other, alternately, creating a pressure differential in said end fluid-passages resulting in a reversing movement of the valve member.

A hydraulic oscillator according to claim 2 in which the pressure actuated-element isa piston and reciprocates in the main chamber which is a cylinder; and the valve member is a spool member which reciprocates in the valve chamber which is cylindrical, the valve member having a pair of axially spaced lands to provide said intermediate fluid passage and being of less axial length than the valve chamber but having axial extensions axially outwardly of the respective lands so as to produce said end fluid-passages in the valve chamber.

4. A hydraulic oscillator comprising a housing having a main pressure inlet and an exhaust outlet, a cylindrical chamber formed within the housing and having a piston slidably mounted therein, said chamber having a first pair of axially spaced pressure inlet ports connected to said main inlet port and being positioned so that both are closed in an intermediate position of said piston but are alternately opened as the piston completes its stroke in opposite directions so as to alternately supply pressure in the chamber at the opposite faces of the piston, said chamber having a second pair of axially spaced exhaust ports which are' closed in an intermediate position of said piston but are alternately opened to opposite ends of said chamber as the piston complestes its stroke in opposite directions, an inlet port and an outlet port at the respective end of the chamber being opened simultaneously as the piston completes its stroke to the opposite end of the chamber, said chamber having a third pair of ports which always connect with the respective ends of the chamber and which serve alternately as inlet and exhaust ports, a reversing valve member mounted for reciprocation within a valve chamber formed within said housing, said valve having a single fluid passage formed intermediate its ends, said main pressure inlet and said first pair of pressure inlet ports for said cylindrical chamber being always connected to said fluid passage of the valve, one of said second pair of axially spaced exhaust ports being constantly connected to one end of said valve chamber and the exhaust outlet and the other being constantly connected to the other end of said valve chamber and the exhaust outlet, said third pair of cylinder end ports being connected to said valve chamber at axially spaced ports which alternately respectively communicate with said fluid passage of the valve as it completes its stroke at one end of the valve chamber and which are both closed when the valve is in an intermediate position.

5. A hydraulic oscillator according to claim 4 including an additional valve for controlling flow of fluid through the housing.

6. An oscillator according to claim 4 in which said piston is free of any passages for hydraulic fluid.

7. A hydraulic oscillator according to claim 4 in which the valve member is in the form of a spool having a pair of axially spaced lands with said fluid passage thereof being a single annular continuous fluid passage formed therebetween, said valve chamber being of sufficient length to provide chambers at each end of the spool, said main pressure inlet and said first pair of pressure inlet ports for said cylindrical chamber being connected to a common port opening into said valve chamber at an intermediate position where it will communicate with said annular passage in all positions of the valve member, said second pair of exhaust ports of the cylinder being connected to the respective end valve chambers by ports opening thereinto, each of said ports also being connected to the main outlet, said third pair of cylinder end ports being connected to the valve chamber at ports opening into the valve chamber at intermediate positions corresponding to the axial spacing of said lands so that they are both closed at an intermediate position of the valve and alternately communicate with said annular valve passage and one of the end chambers of the valve.

8. A hydraulic oscillator according to claim 7 in which the cylinder and valve chamber are in substantial axial alignment, said piston in said cylinder having an actuating rod extending from the housing.

9. A hydraulic oscillator according to claim 7 in which the cylinder and valve chamber have their axes in parallel relationship, said piston having actuating rods extending axially in opposite directions therefrom through opposed walls of the housing. 

1. A hydraulic oscillator comprising two movable elements only consisting of a reciprocable piston in a cylinder and a reciprocable reversing valve in a chamber, means for creating differential flow conditions at the opposite ends of the valve in the valve chamber, said means including inlet and outlet ports in the cylinder which are connected to the valve chamber and are covered and uncovered by the piston during its reciprocation, said movable elements disposed in a housing having a main inlet and outlet with the piston reciprocable in the cylinder formed in said housing and the valve member reciprocable in the chamber formed in said housing, said valve member having a single fluid passage formed intermediate its ends, said valve member fluid passage being constantly connected to said main inlet, said valve chamber having opposed ends constantly connected to said main outlet, a passage between said valve fluid passage and one end of said cylinder and a passage between one end of said valve chamber and the other end of said cylinder, said valve member in its extreme end positions serving to connect alternately the main inlet to one of said passages and disconnecte the other therefrom, the disconnected passage being connected to one end of the valve chamber and to said outlet to create a dynamic pressure differential thereon to hold it in said extreme position, and a pair of cooperating inlet and outlet ports in each end of the cylinder alternately exposed and closed by the piston in its stroke in opposite directions for creating at the end of each stroke an alternate branch passage from the main inlet, through the valve member fluid passage and the end of the cylindeR with the exposed ports to the opposite end of the valve chamber to create a dynamic differential pressure thereon to cause it to move in a reverse direction.
 2. A hydraulic oscillator comprising two movable elements consisting of a main movable pressure-actuated element and a movable control valve member located in a main chamber and a valve chamber respectively; said main movable member disposed in its chamber for movement therein and dividing the main chamber into opposed end chambers; said movable valve member disposed in its chamber and of such form as to divide the valve chamber into a single intermediate fluid-passage chamber and opposed end fluid-passage chambers; a main pressure inlet passage always connected to the intermediate fluid-passage chamber of the valve member in all its positions of movement; a main outlet passage always connected to both the opposed end fluid-passage chambers of the valve chamber; and fluid passages connecting the end chambers of the main chamber and the end fluid-passages of the valve chamber under the control of said pressure-actuated element to create dynamic flow conditions in one of said end fluid-passage chambers of the valve and static conditions in the other, alternately, creating a pressure differential in said end fluid-passages resulting in a reversing movement of the valve member.
 3. A hydraulic oscillator according to claim 2 in which the pressure actuated element is a piston and reciprocates in the main chamber which is a cylinder; and the valve member is a spool member which reciprocates in the valve chamber which is cylindrical, the valve member having a pair of axially spaced lands to provide said intermediate fluid passage and being of less axial length than the valve chamber but having axial extensions axially outwardly of the respective lands so as to produce said end fluid-passages in the valve chamber.
 4. A hydraulic oscillator comprising a housing having a main pressure inlet and an exhaust outlet, a cylindrical chamber formed within the housing and having a piston slidably mounted therein, said chamber having a first pair of axially spaced pressure inlet ports connected to said main inlet port and being positioned so that both are closed in an intermediate position of said piston but are alternately opened as the piston completes its stroke in opposite directions so as to alternately supply pressure in the chamber at the opposite faces of the piston, said chamber having a second pair of axially spaced exhaust ports which are closed in an intermediate position of said piston but are alternately opened to opposite ends of said chamber as the piston complestes its stroke in opposite directions, an inlet port and an outlet port at the respective end of the chamber being opened simultaneously as the piston completes its stroke to the opposite end of the chamber, said chamber having a third pair of ports which always connect with the respective ends of the chamber and which serve alternately as inlet and exhaust ports, a reversing valve member mounted for reciprocation within a valve chamber formed within said housing, said valve having a single fluid passage formed intermediate its ends, said main pressure inlet and said first pair of pressure inlet ports for said cylindrical chamber being always connected to said fluid passage of the valve, one of said second pair of axially spaced exhaust ports being constantly connected to one end of said valve chamber and the exhaust outlet and the other being constantly connected to the other end of said valve chamber and the exhaust outlet, said third pair of cylinder end ports being connected to said valve chamber at axially spaced ports which alternately respectively communicate with said fluid passage of the valve as it completes its stroke at one end of the valve chamber and which are both closed when the valve is in an intermediate position.
 5. A hydraulic oscillator according to claim 4 including an additional valve for controlling flow of fluid through the houSing.
 6. An oscillator according to claim 4 in which said piston is free of any passages for hydraulic fluid.
 7. A hydraulic oscillator according to claim 4 in which the valve member is in the form of a spool having a pair of axially spaced lands with said fluid passage thereof being a single annular continuous fluid passage formed therebetween, said valve chamber being of sufficient length to provide chambers at each end of the spool, said main pressure inlet and said first pair of pressure inlet ports for said cylindrical chamber being connected to a common port opening into said valve chamber at an intermediate position where it will communicate with said annular passage in all positions of the valve member, said second pair of exhaust ports of the cylinder being connected to the respective end valve chambers by ports opening thereinto, each of said ports also being connected to the main outlet, said third pair of cylinder end ports being connected to the valve chamber at ports opening into the valve chamber at intermediate positions corresponding to the axial spacing of said lands so that they are both closed at an intermediate position of the valve and alternately communicate with said annular valve passage and one of the end chambers of the valve.
 8. A hydraulic oscillator according to claim 7 in which the cylinder and valve chamber are in substantial axial alignment, said piston in said cylinder having an actuating rod extending from the housing.
 9. A hydraulic oscillator according to claim 7 in which the cylinder and valve chamber have their axes in parallel relationship, said piston having actuating rods extending axially in opposite directions therefrom through opposed walls of the housing. 